Triangulation scanner and camera for augmented reality

ABSTRACT

A method of combining 2D images into a 3D image includes providing a coordinate measurement device and a triangulation scanner having an integral camera associated therewith, the scanner being separate from the coordinate measurement device. In a first instance, the coordinate measurement device determines the position and orientation of the scanner and the integral camera captures a first 2D image. In a second instance, the scanner is moved, the coordinate measurement device determines the position and orientation of the scanner, and the integral camera captures a second 2D image. A cardinal point common to the first and second images is found and is used, together with the first and second images and the positions and orientations of the scanner in the first and second instances, to create the 3D image.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 14/960,626 filed on Dec. 7, 2015, which is anonprovisional application of U.S. Provisional Application Ser. No.62/092,371, filed on Dec. 16, 2014, the contents of both of which areincorporated herein by reference.

BACKGROUND

The present disclosure relates to a triangulation scanner and anintegral camera configured to achieve augmented reality (AR).

One set of coordinate measurement devices belongs to a class ofinstruments that measure the three-dimensional (3D) coordinates of apoint by sending a laser beam to the point. The laser beam may impingedirectly on the point or on a retroreflector target in contact with thepoint. In either case, the instrument determines the coordinates of thepoint by measuring the distance and the two angles to the target. Thedistance is measured with a distance measuring device such as anabsolute distance meter or an interferometer. The angles are measuredwith an angle measuring device such as an angular encoder. A gimbaledbeam-steering mechanism within the instrument directs the laser beam tothe point of interest.

The laser tracker is a particular type of coordinate measuring devicethat tracks the retroreflector target with one or more laser beams itemits. The laser tracker is thus a “time-of-flight” type of measurementdevice. Coordinate measuring devices closely related to the lasertracker are the laser scanner and the total station. The laser scannersteps one or more laser beams to points on a surface of an object. Itpicks up light scattered from the surface and from this light determinesthe distance and two angles to each point. The total station, which ismost often used in surveying applications, may be used to measure thecoordinates of diffusely scattering or retroreflective targets.Hereinafter, the term laser tracker is used in a broad sense to includelaser scanners and total stations.

Ordinarily the laser tracker sends a laser beam to a retroreflectortarget. A common type of retroreflector target is the sphericallymounted retroreflector (SMR), which comprises a cube-cornerretroreflector embedded within a metal sphere. The cube-cornerretroreflector comprises three mutually perpendicular mirrors. Thevertex, which is the common point of intersection of the three mirrors,is located at the center of the sphere. Because of this placement of thecube corner within the sphere, the perpendicular distance from thevertex to any surface on which the SMR rests remains constant, even asthe SMR is rotated. Consequently, the laser tracker can measure the 3Dcoordinates of the object surface by following the position of an SMR asit is moved over the surface. Stating this another way, the lasertracker needs to measure only three degrees of freedom (one radialdistance and two angles) to fully characterize the 3D coordinates of asurface.

One type of laser tracker contains only an interferometer (IFM) withoutan absolute distance meter (ADM). If an object blocks the path of thelaser beam from one of these trackers, the IFM loses its distancereference. The operator must then track the retroreflector to a knownlocation to reset to a reference distance before continuing themeasurement. A way around this limitation is to put an ADM in thetracker. The ADM can measure distance in a point-and-shoot manner, asdescribed in more detail below. Some laser trackers contain only an ADMwithout an interferometer. U.S. Pat. No. 7,352,446 ('446) to Bridges etal., the contents of which are incorporated herein by reference,describes a laser tracker having only an ADM (and no IFM) that is ableto accurately scan a moving target. Prior to the '446 patent, absolutedistance meters were too slow to accurately find the position of amoving target.

A gimbal mechanism within the laser tracker may be used to direct alaser beam from the tracker to the SMR. Part of the light retroreflectedby the SMR enters the laser tracker and passes onto a position detector.A control system within the laser tracker can use the position of thelight on the position detector to adjust the rotation angles of themechanical axes of the laser tracker to keep the laser beam centered onthe SMR. In this way, the tracker is able to follow (track) an SMR thatis moved over the surface of an object of interest.

Angle measuring devices such as angular encoders are attached to themechanical axes of the tracker. The one distance measurement and twoangle measurements performed by the laser tracker are sufficient tocompletely specify the three-dimensional location of the SMR at anypoint on the surface of the object being measured.

Several laser trackers have been disclosed for measuring six, ratherthan the ordinary three, degrees of freedom. These six degrees offreedom include three translational degrees of freedom and threeorientational degrees of freedom. Exemplary six degree-of-freedom(six-DOF or 6DOF) laser tracker systems are described by U.S. Pat. No.7,800,758 ('758) to Bridges et al., U.S. Pat. No. 8,525,983 ('983) toBridges et al., and U.S. Pat. No. 8,467,072 ('072) to Cramer et al., thecontents of each of which are incorporated herein by reference.

An alternative to a time-of-flight measurement device such as a lasertracker is a scanning system that determines the 3D coordinates of anobject surface based on triangulation principles. Systems such as lasertrackers that make use of time-of-flight distance meters in some casesare relatively more accurate than triangulation scanners, butnon-contact triangulation scanners may be relatively faster because theytypically project a plurality of light spots onto the object surface ateach instant in time.

A typical triangulation scanner projects either a line of light (e.g.,light from a laser line probe) or a two-dimensional (2D) pattern oflight over an area (e.g., structured light) onto the object surface. Ina triangulation scanner, a camera (e.g., a CCD or CMOS photosensitivearray) is coupled to a projector (e.g., a laser light source) in a fixedmechanical relationship. The projected line or pattern of light emittedfrom the projector is reflected off of the object surface and imaged bythe camera. Since the camera and projector are arranged in a fixedrelationship to each other, the distance and angles to the objectsurface may be determined from the projected line or pattern, thecaptured camera images and a baseline distance separating the projectorand the camera according to trigonometric principles. Triangulationsystems provide advantages in quickly acquiring 3D coordinate data overlarge areas.

In some systems, during the scanning process, the triangulation scanneracquires a series of 3D images, which may be registered relative to eachother so that the position and orientation of each 3D image relative tothe other 3D images is known. If the scanner is stationary, such imageregistration is not necessary. Similarly, if the triangulation scanneris attached to, or works in conjunction with, a mechanical device havingthe ability to measure the position and orientation of the triangulationscanner, it is not necessary to provide such image registration.Examples of such mechanical devices include laser trackers, articulatedarm coordinate measurement machines (CMMs), and Cartesian CMMs.

On the other hand, where the scanner is handheld and hence movable,various techniques may be used to register the images. One commontechnique uses features (e.g., cardinal points) located in the images tomatch overlapping areas of adjacent image frames. This technique workswell when the object being measured has many features relative to thefield of view of the scanner. However, if the object contains arelatively large flat or curved surface, the images may not properlyregister relative to each other.

Accordingly, while existing coordinate measurement devices are suitablefor their intended purposes in working with triangulation scanners asdescribed hereinabove, the need for improvement remains, particularly inimproving the registration of images acquired by a triangulation scannerdevice.

Augmented reality (AR) is a relatively new type of technology that grewout of virtual reality. Augmented reality merges, superimposes, ortransprojects actual real-world information or data with, on, into, oronto virtual information or data. That is, the virtual information ordata “augments,” compliments or supplements the actual sensed, measured,captured or imaged real-world information or data related to some objector scene to give the user an enhanced view or perception of the realworld object or scene. Augmented reality applications include technicalor industrial areas such as part, component or device manufacturing andassembly and/or repair and maintenance, and facility, building orstructure layout and construction. A number of modern-day ARapplications are disclosed athttp://en.wikipedia.org/wiki/Augmented_reality.

The actual information or data relating to the part, component or deviceor area may be obtained in various ways using various devices. One typeof device includes a coordinate measurement device such as, for example,a CMM or a laser tracker. A camera may also be used to take still orvideo images of the actual part, component or device, and/or a desiredarea by itself or that surrounding or associated with the part,component or device.

The virtual information or data may be stored artificial informationregarding the part, component or device. The stored virtual informationor data may be related to the design of the part, component or deviceranging from, for example, simple text or symbols to relatively morecomplex, graphic 3D CAD design data. Besides visual information, thestored virtual information or data may also comprise audible or soundinformation or data. The stored virtual information or data may alsorelate to information such as textual or part, component or devicerepair or maintenance instructions, or visual information depictingparts, components or devices that may be used, for example, in thedesign of an office or manufacturing and/or repair facility (e.g., abuilding or facility layout).

The combined actual and virtual information or data in an AR system isusually digital in nature and may be delivered in real-time (i.e., asthe actual information is being measured or sensed) to a user on adisplay screen that may be in many different types or forms, such asthat associated with, for example, a desktop or laptop computer monitor,tablet, smartphone or even a head-mounted display such as thoseassociated with glasses, hats or helmets. Audio information may bedelivered through a speaker.

While some innovations have already been made in the area of augmentedreality for use with various types of devices, there is a need for novelapplications of augmented reality together with handheld triangulationscanners (e.g., structured light scanners, laser line probes) used witha laser tracker.

SUMMARY

In an embodiment of the present invention, a method of combining aplurality of two-dimensional (2D) images into a three-dimensional (3D)image comprises steps of: providing a six-degree of freedom (six-DOF)triangulation scanner, the triangulation scanner including aretroreflector and an integral augmented reality (AR) camera; providinga coordinate measurement device having a device frame of reference, thedevice being separate from the triangulation scanner, the deviceincluding a first motor, a second motor, a first angle measuringtransducer, a second angle measuring transducer, a distance meter, aposition detector, a control system, and a processor, the first motorand the second motor configured together to direct a first beam of lightto a first direction, the first direction determined by a first angle ofrotation about a first axis and a second angle of rotation about asecond axis, the first angle of rotation produced by the first motor andthe second angle of rotation produced by the second motor, the firstangle measuring transducer configured to measure the first angle ofrotation and the second angle measuring transducer configured to measurethe second angle of rotation, the distance meter configured to measure adistance from the device to the retroreflector based at least in part ona first part of the first beam of light reflected by the retroreflectorand received by a first optical detector and on a speed of light in air,the position detector configured to receive a second part of the firstbeam of light reflected by the retroreflector and to produce a firstsignal in response, the control system configured to send a secondsignal to the first motor and a third signal to the second motor, thesecond signal and the third signal based at least in part on the firstsignal, the control system configured to adjust the first direction ofthe first beam of light to a position in space of the retroreflector,the processor configured to determine, in the device frame of reference,3D translational coordinates of the triangulation scanner; in a firstinstance: with the device, measuring a third angle with the first anglemeasuring transducer, measuring a fourth angle with the second anglemeasuring transducer, measuring with the distance meter a firstdistance; forming a first 2D image with the camera; in a secondinstance: moving the triangulation scanner; with the device, measuring afifth angle with the first angle measuring transducer, measuring a sixthangle with the second angle measuring transducer, measuring with thedistance meter a second distance; forming a second 2D image with thecamera; determining a first cardinal point in common between the firstand second 2D images, the first cardinal point having a first locationon the first 2D image and a second location on the second 2D image;determining 3D coordinates of the first cardinal point in a first frameof reference based at least in part on the third angle, the fourthangle, the fifth angle, the sixth angle, the first distance, the seconddistance, the first location, and the second location; creating the 3Dimage as a first composite 3D image from the first 2D image and thesecond 2D image based at least in part on the first 2D image, the second2D image, and the 3D coordinates of the first cardinal point in thefirst frame of reference; and storing the first composite 3D image.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, exemplary embodiments are shown whichshould not be construed to be limiting regarding the entire scope of thedisclosure, and wherein the elements are numbered alike in severalFIGURES:

FIG. 1 is a perspective view of a laser tracker system with aretroreflector target in accordance with an embodiment of the presentinvention;

FIG. 2 is a perspective view of a laser tracker system with a six-degreeof freedom (six-DOF) target;

FIG. 3 is a block diagram describing elements of laser tracker opticsand electronics in accordance with an embodiment of the presentinvention;

FIG. 4, which includes FIGS. 4A and 4B, shows two types of prior artafocal beam expanders;

FIG. 5 shows a prior art fiber-optic beam launch;

FIGS. 6A-D are schematic figures that show four types of prior artposition detector assemblies;

FIGS. 6E and 6F are schematic figures showing position detectorassemblies according to embodiments of the present invention;

FIG. 7 is a block diagram of electrical and electro-optical elementswithin a prior art ADM;

FIGS. 8A and 8B are schematic figures showing fiber-optic elementswithin a prior art fiber-optic network;

FIG. 8C is a schematic figure showing fiber-optic elements within afiber-optic network in accordance with an embodiment of the presentinvention;

FIG. 9 is an exploded view of a prior art laser tracker;

FIG. 10 is a cross-sectional view of a prior art laser tracker;

FIG. 11 is a block diagram of the computing and communication elementsof a laser tracker in accordance with an embodiment of the presentinvention;

FIG. 12A is a block diagram of elements in a laser tracker that uses asingle wavelength according to an embodiment of the present invention;

FIG. 12B is a block diagram of elements in a laser tracker that uses asingle wavelength according to an embodiment of the present invention;

FIG. 13 is a block diagram of elements in a laser tracker;

FIG. 14 is a block diagram of elements in a laser tracker according toan embodiment of the present invention;

FIGS. 15A and 15B are schematic representations illustrating theprinciples of operation of triangulation based scanning measurementsystems; and

FIG. 16 is a flow chart that includes steps in a method of obtaining athree-dimensional representation of a surface using an augmented realitycamera attached to a triangulation scanner according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

An exemplary laser tracker system 5 illustrated in FIG. 1 includes alaser tracker 10, a retroreflector target 26, an optional auxiliary unitprocessor 50, and an optional auxiliary computer 60. An exemplarygimbaled beam-steering mechanism 12 of laser tracker 10 comprises azenith carriage 14 mounted on an azimuth base 16 and rotated about anazimuth axis 20. A payload 15 is mounted on the zenith carriage 14 androtated about a zenith axis 18. Zenith axis 18 and azimuth axis 20intersect orthogonally, internally to tracker 10, at gimbal point 22,which is typically the origin for distance measurements. A laser beam 46virtually passes through the gimbal point 22 and is pointed orthogonalto zenith axis 18. In other words, laser beam 46 lies in a planeapproximately perpendicular to the zenith axis 18 and that passesthrough the azimuth axis 20. Outgoing laser beam 46 is pointed in thedesired direction by rotation of payload 15 about zenith axis 18 and byrotation of zenith carriage 14 about azimuth axis 20. A zenith angularencoder, internal to the tracker, is attached to a zenith mechanicalaxis aligned to the zenith axis 18. An azimuth angular encoder, internalto the tracker, is attached to an azimuth mechanical axis aligned to theazimuth axis 20. The zenith and azimuth angular encoders measure thezenith and azimuth angles of rotation to relatively high accuracy.Outgoing laser beam 46 travels to the retroreflector target 26, whichmight be, for example, a spherically mounted retroreflector (SMR) asdescribed above. By measuring the radial distance between gimbal point22 and retroreflector 26, the rotation angle about the zenith axis 18,and the rotation angle about the azimuth axis 20, the position ofretroreflector 26 is found within the spherical coordinate system of thetracker.

Outgoing laser beam 46 may include one or more laser wavelengths, asdescribed hereinafter. For the sake of clarity and simplicity, asteering mechanism of the sort shown in FIG. 1 is assumed in thefollowing discussion. However, other types of steering mechanisms arepossible. For example, it is possible to reflect a laser beam off amirror rotated about the azimuth and zenith axes. The techniquesdescribed herein are applicable, regardless of the type of steeringmechanism.

Magnetic nests 17 may be included on the laser tracker for resetting thelaser tracker to a “home” position for different sized SMRs—for example,1.5, ⅞, and ½ inch SMRs. An on-tracker retroreflector 19 may be used toreset the tracker to a reference distance. In addition, an on-trackermirror, not visible from the view of FIG. 1, may be used in combinationwith the on-tracker retroreflector to enable performance of aself-compensation, as described in U.S. Pat. No. 7,327,446 ('446), thecontents of which are incorporated herein by reference.

FIG. 2 shows an exemplary laser tracker system 7 that is like the lasertracker system 5 of FIG. 1 except that retroreflector target 26 isreplaced with a six-DOF probe 1000. In FIG. 1, other types ofretroreflector targets may be used. For example, a cateyeretroreflector, which is a glass retroreflector in which light focusesto a small spot of light on a reflective rear surface of the glassstructure, is sometimes used.

FIG. 3 is a block diagram showing optical and electrical elements in alaser tracker embodiment. It shows elements of a laser tracker that emittwo wavelengths of light—a first wavelength for an ADM and a secondwavelength for a visible pointer and for tracking. The visible pointerenables the user to see the position of the laser beam spot emitted bythe tracker. The two different wavelengths are combined using afree-space beam splitter. Electrooptic (EO) system 100 includes visiblelight source 110, isolator 115, optional first fiber launch 170,optional interferometer (IFM) 120, beam expander 140, first beamsplitter 145, position detector assembly 150, second beam splitter 155,ADM 160, and second fiber launch 170.

Visible light source 110 may be a laser, superluminescent diode, orother light emitting device. The isolator 115 may be a Faraday isolator,attenuator, or other device capable of reducing the light that reflectsback into the light source. Optional IFM may be configured in a varietyof ways. As a specific example of a possible implementation, the IFM mayinclude a beam splitter 122, a retroreflector 126, quarter waveplates124, 130, and a phase analyzer 128. The visible light source 110 maylaunch the light into free space, the light then traveling in free spacethrough the isolator 115, and optional IFM 120. Alternatively, theisolator 115 may be coupled to the visible light source 110 by a fiberoptic cable. In this case, the light from the isolator may be launchedinto free space through the first fiber-optic launch 170, as discussedhereinbelow with reference to FIG. 5.

Beam expander 140 may be set up using a variety of lens configurations,but two commonly used prior-art configurations are shown in FIGS. 4A and4B. FIG. 4A shows a configuration 140A based on the use of a negativelens 141A and a positive lens 142A. A beam of collimated light 220Aincident on the negative lens 141A emerges from the positive lens 142Aas a larger beam of collimated light 230A. FIG. 4B shows a configuration140B based on the use of two positive lenses 141B, 142B. A beam ofcollimated light 220B incident on a first positive lens 141B emergesfrom a second positive lens 142B as a larger beam of collimated light230B. Of the light leaving the beam expander 140, a small amountreflects off the beam splitters 145, 155 on the way out of the trackerand is lost. That part of the light that passes through the beamsplitter 155 is combined with light from the ADM 160 to form a compositebeam of light 188 that leaves that laser tracker and travels to theretroreflector 90.

In an embodiment, the ADM 160 includes a light source 162, ADMelectronics 164, a fiber network 166, an interconnecting electricalcable 165, and interconnecting optical fibers 168, 169, 184, 186. ADMelectronics send electrical modulation and bias voltages to light source162, which may, for example, be a distributed feedback laser thatoperates at a wavelength of approximately 1550 nm. In an embodiment, thefiber network 166 may be the prior art fiber-optic network 420A shown inFIG. 8A. In this embodiment, light from the light source 162 in FIG. 3travels over the optical fiber 184, which is equivalent to the opticalfiber 432 in FIG. 8A.

The fiber network of FIG. 8A includes a first fiber coupler 430, asecond fiber coupler 436, and low-transmission reflectors 435, 440. Thelight travels through the first fiber coupler 430 and splits between twopaths, the first path through optical fiber 433 to the second fibercoupler 436 and the second path through optical fiber 422 and fiberlength equalizer 423. Fiber length equalizer 423 connects to fiberlength 168 in FIG. 3, which travels to the reference channel of the ADMelectronics 164. The purpose of fiber length equalizer 423 is to matchthe length of optical fibers traversed by light in the reference channelto the length of optical fibers traversed by light in the measurechannel. Matching the fiber lengths in this way reduces ADM errorscaused by changes in the ambient temperature. Such errors may arisebecause the effective optical path length of an optical fiber is equalto the average index of refraction of the optical fiber times the lengthof the fiber. Since the index of refraction of the optical fibersdepends on the temperature of the fiber, a change in the temperature ofthe optical fibers causes changes in the effective optical path lengthsof the measure and reference channels. If the effective optical pathlength of the optical fiber in the measure channel changes relative tothe effective optical path length of the optical fiber in the referencechannel, the result will be an apparent shift in the position of theretroreflector target 90, even if the retroreflector target 90 is keptstationary. To get around this problem, two steps are taken. First, thelength of the fiber in the reference channel is matched, as nearly aspossible, to the length of the fiber in the measure channel. Second, themeasure and reference fibers are routed side by side to the extentpossible to ensure that the optical fibers in the two channels seenearly the same changes in temperature.

The light travels through the second fiber optic coupler 436 and splitsinto two paths, the first path to the low-reflection fiber terminator440 and the second path to optical fiber 438, from which it travels tooptical fiber 186 in FIG. 3. The light on optical fiber 186 travelsthrough to the second fiber launch 170.

In an embodiment, fiber launch 170 is shown in prior art FIG. 5. Thelight from optical fiber 186 of FIG. 3 goes to fiber 172 in FIG. 5. Thefiber launch 170 includes optical fiber 172, ferrule 174, and lens 176.The optical fiber 172 is attached to ferrule 174, which is stablyattached to a structure within the laser tracker 10. If desired, the endof the optical fiber may be polished at an angle to reduce backreflections. The light 250 emerges from the core of the fiber, which maybe a single mode optical fiber with a diameter of between 4 and 12micrometers, depending on the wavelength of the light being used and theparticular type of optical fiber. The light 250 diverges at an angle andintercepts lens 176, which collimates it. The method of launching andreceiving an optical signal through a single optical fiber in an ADMsystem was described in reference to FIG. 3 in patent '758.

Referring to FIG. 3, the beam splitter 155 may be a dichroic beamsplitter, which transmits different wavelengths than it reflects. In anembodiment, the light from the ADM 160 reflects off dichroic beamsplitter 155 and combines with the light from the visible laser 110,which is transmitted through the dichroic beam splitter 155. Thecomposite beam of light 188 travels out of the laser tracker toretroreflector 90 as a first beam, which returns a portion of the lightas a second beam. That portion of the second beam that is at the ADMwavelength reflects off the dichroic beam splitter 155 and returns tothe second fiber launch 170, which couples the light back into theoptical fiber 186.

In an embodiment, the optical fiber 186 corresponds to the optical fiber438 in FIG. 8A. The returning light travels from optical fiber 438through the second fiber coupler 436 and splits between two paths. Afirst path leads to optical fiber 424 that, in an embodiment,corresponds to optical fiber 169 that leads to the measure channel ofthe ADM electronics 164 in FIG. 3. A second path leads to optical fiber433 and then to the first fiber coupler 430. The light leaving the firstfiber coupler 430 splits between two paths, a first path to the opticalfiber 432 and a second path to the low reflectance termination 435. Inan embodiment, optical fiber 432 corresponds to the optical fiber 184,which leads to the light source 162 in FIG. 3. In most cases, the lightsource 162 contains a built-in Faraday isolator that minimizes theamount of light that enters the light source from optical fiber 432.Excessive light fed into a laser in the reverse direction candestabilize the laser.

The light from the fiber network 166 enters ADM electronics 164 throughoptical fibers 168, 169. An embodiment of prior art ADM electronics isshown in FIG. 7. Optical fiber 168 in FIG. 3 corresponds to opticalfiber 3232 in FIG. 7, and optical fiber 169 in FIG. 3 corresponds tooptical fiber 3230 in FIG. 7. Referring now to FIG. 7, ADM electronics3300 includes a frequency reference 3302, a synthesizer 3304, a measuredetector 3306, a reference detector 3308, a measure mixer 3310, areference mixer 3312, conditioning electronics 3314, 3316, 3318, 3320, adivide-by-N prescaler 3324, and an analog-to-digital converter (ADC)3322. The frequency reference, which might be an oven-controlled crystaloscillator (OCXO), for example, sends a reference frequency f_(REF),which might be 10 MHz, for example, to the synthesizer, which generatestwo electrical signals—one signal at a frequency f_(RF) and two signalsat frequency f_(LO). The signal f_(RF) goes to the light source 3102,which corresponds to the light source 162 in FIG. 3. The two signals atfrequency f_(LO) go to the measure mixer 3310 and the reference mixer3312. The light from optical fibers 168, 169 in FIG. 3 appear on fibers3232, 3230 in FIG. 7, respectively, and enter the reference and measurechannels, respectively. Reference detector 3308 and measure detector3306 convert the optical signals into electrical signals. These signalsare conditioned by electrical components 3316, 3314, respectively, andare sent to mixers 3312, 3310, respectively. The mixers produce afrequency f_(IF) equal to the absolute value of f_(LO)−f_(RF). Thesignal f_(RF) may be a relatively high frequency, for example, 2 GHz,while the signal f_(IF) may have a relatively low frequency, forexample, 10 kHz.

The reference frequency f_(REF) is sent to the prescaler 3324, whichdivides the frequency by an integer value. For example, a frequency of10 MHz might be divided by 40 to obtain an output frequency of 250 kHz.In this example, the 10 kHz signals entering the ADC 3322 would besampled at a rate of 250 kHz, thereby producing 25 samples per cycle.The signals from the ADC 3322 are sent to a data processor 3400, whichmight, for example, be one or more digital signal processor (DSP) unitslocated in ADM electronics 164 of FIG. 3.

The method for extracting a distance is based on the calculation ofphase of the ADC signals for the reference and measure channels. Thismethod is described in detail in U.S. Pat. No. 7,701,559 ('559) toBridges et al., the contents of which are incorporated herein byreference. Calculation includes use of equations (1)-(8) of patent '559.In addition, when the ADM first begins to measure a retroreflector, thefrequencies generated by the synthesizer are changed some number oftimes (for example, three times), and the possible ADM distancescalculated in each case. By comparing the possible ADM distances foreach of the selected frequencies, an ambiguity in the ADM measurement isremoved. The equations (1)-(8) of patent '559 combined withsynchronization methods described with respect to FIG. 5 of patent '559and the Kalman filter methods described in patent '559 enable the ADM tomeasure a moving target. In other embodiments, other methods ofobtaining absolute distance measurements, for example, by using pulsedtime-of-flight rather than phase differences, may be used.

The part of the return light beam 190 that passes through the beamsplitter 155 arrives at the beam splitter 145, which sends part of thelight to the beam expander 140 and another part of the light to theposition detector assembly 150. The light emerging from the lasertracker 10 or EO system 100 may be thought of as a first beam and theportion of that light reflecting off the retroreflector 90 or 26 as asecond beam. Portions of the reflected beam are sent to differentfunctional elements of the EO system 100. For example, a first portionmay be sent to a distance meter such as an ADM 160 in FIG. 3. A secondportion may be sent to a position detector assembly 150. In some cases,a third portion may be sent to other functional units such as anoptional interferometer 120. It is important to understand that,although, in the example of FIG. 3, the first portion and the secondportion of the second beam are sent to the distance meter and theposition detector after reflecting off beam splitters 155 and 145,respectively, it would have been possible to transmit, rather thanreflect, the light onto a distance meter or position detector.

Four examples of prior art position detector assemblies 150A-150D areshown in FIGS. 6A-D. FIG. 6A depicts the simplest implementation, withthe position detector assembly including a position sensor 151 mountedon a circuit board 152 that obtains power from and returns signals toelectronics box 350, which may represent electronic processingcapability at any location within the laser tracker 10, auxiliary unit50, or external computer 60. FIG. 6B includes an optical filter 154 thatblocks unwanted optical wavelengths from reaching the position sensor151. The unwanted optical wavelengths may also be blocked, for example,by coating the beam splitter 145 or the surface of the position sensor151 with an appropriate film. FIG. 6C includes a lens 153 that reducesthe size of the beam of light. FIG. 6D includes both an optical filter154 and a lens 153.

FIG. 6E shows a novel position detector assembly that includes anoptical conditioner 149E. Optical conditioner contains a lens 153 andmay also contain optional wavelength filter 154. In addition, itincludes at least one of a diffuser 156 and a spatial filter 157. Asexplained hereinabove, a popular type of retroreflector is thecube-corner retroreflector. One type of cube corner retroreflector ismade of three mirrors, each joined at right angles to the other twomirrors. Lines of intersection at which these three mirrors are joinedmay have a finite thickness in which light is not perfectly reflectedback to the tracker. The lines of finite thickness are diffracted asthey propagate so that upon reaching the position detector they may notappear exactly the same as at the position detector. However, thediffracted light pattern will generally depart from perfect symmetry. Asa result, the light that strikes the position detector 151 may have, forexample, dips or rises in optical power (hot spots) in the vicinity ofthe diffracted lines. Because the uniformity of the light from theretroreflector may vary from retroreflector to retroreflector and alsobecause the distribution of light on the position detector may vary asthe retroreflector is rotated or tilted, it may be advantageous toinclude a diffuser 156 to improve the smoothness of the light thatstrikes the position detector 151. It might be argued that, because anideal position detector should respond to a centroid and an idealdiffuser should spread a spot symmetrically, there should be no effecton the resulting position given by the position detector. However, inpractice the diffuser is observed to improve performance of the positiondetector assembly, probably because the effects of nonlinearities(imperfections) in the position detector 151 and the lens 153. Cubecorner retroreflectors made of glass may also produce non-uniform spotsof light at the position detector 151. In an embodiment, the diffuser156 is a holographic diffuser. A holographic diffuser providescontrolled, homogeneous light over a specified diffusing angle. In otherembodiments, other types of diffusers such as ground glass or “opal”diffusers are used.

The purpose of the spatial filter 157 of the position detector assembly150E is to block ghost beams that may be the result, for example, ofunwanted reflections off optical surfaces, from striking the positiondetector 151. A spatial filter includes a plate 157 that has anaperture. By placing the spatial filter 157 a distance away from thelens equal approximately to the focal length of the lens, the returninglight 243E passes through the spatial filter when it is near itsnarrowest—at the waist of the beam. Beams that are traveling at adifferent angle, for example, as a result of reflection of an opticalelement strike the spatial filter away from the aperture and are blockedfrom reaching the position detector 151. An example is shown in FIG. 6E,where an unwanted ghost beam 244E reflects off a surface of the beamsplitter 145 and travels to spatial filter 157, where it is blocked.Without the spatial filter, the ghost beam 244E would have interceptedthe position detector 151, thereby causing the position of the beam 243Eon the position detector 151 to be incorrectly determined. Even a weakghost beam may significantly change the position of the centroid on theposition detector 151 if the ghost beam is located a relatively largedistance from the main spot of light.

A retroreflector of the sort discussed here, a cube corner or a cateyeretroreflector, for example, has the property of reflecting a ray oflight that enters the retroreflector in a direction parallel to theincident ray. In addition, the incident and reflected rays aresymmetrically placed about the point of symmetry of the retroreflector.For example, in an open-air cube corner retroreflector, the point ofsymmetry of the retroreflector is the vertex of the cube corner. In aglass cube corner retroreflector, the point of symmetry is also thevertex, but one must consider the bending of the light at the glass-airinterface in this case. In a cateye retroreflector having an index ofrefraction of 2.0, the point of symmetry is the center of the sphere. Ina cateye retroreflector made of two glass hemispheres symmetricallyseated on a common plane, the point of symmetry is a point lying on theplane and at the spherical center of each hemisphere. The main point isthat, for the type of retroreflectors ordinarily used with lasertrackers, the light returned by a retroreflector to the tracker isshifted to the other side of the vertex relative to the incident laserbeam.

This behavior of a retroreflector 90 in FIG. 3 is the basis for thetracking of the retroreflector by the laser tracker. The position sensorhas on its surface an ideal retrace point. The ideal retrace point isthe point at which a laser beam sent to the point of symmetry of aretroreflector (e.g., the vertex of the cube corner retroreflector in anSMR) will return. Usually the retrace point is near the center of theposition sensor. If the laser beam is sent to one side of theretroreflector, it reflects back on the other side and appears off theretrace point on the position sensor. By noting the position of thereturning beam of light on the position sensor, the control system ofthe laser tracker 10 can cause the motors to move the light beam towardthe point of symmetry of the retroreflector.

If the retroreflector is moved transverse to the tracker at a constantvelocity, the light beam at the retroreflector will strike theretroreflector (after transients have settled) a fixed offset distancefrom the point of symmetry of the retroreflector. The laser trackermakes a correction to account for this offset distance at theretroreflector based on scale factor obtained from controlledmeasurements and based on the distance from the light beam on theposition sensor to the ideal retrace point.

As explained hereinabove, the position detector performs two importantfunctions—enabling tracking and correcting measurements to account forthe movement of the retroreflector. The position sensor within theposition detector may be any type of device capable of measuring aposition. For example, the position sensor might be a position sensitivedetector or a photosensitive array. The position sensitive detectormight be lateral effect detector or a quadrant detector, for example.The photosensitive array might be a CMOS or CCD array, for example.

In an embodiment, the return light that does not reflect off beamsplitter 145 passes through beam expander 140, thereby becoming smaller.In another embodiment, the positions of the position detector and thedistance meter are reversed so that the light reflected by the beamsplitter 145 travels to the distance meter and the light transmitted bythe beam splitter travels to the position detector.

The light continues through optional IFM, through the isolator and intothe visible light source 110. At this stage, the optical power should besmall enough so that it does not destabilize the visible light source110.

In an embodiment, the light from visible light source 110 is launchedthrough a beam launch 170 of FIG. 5. The fiber launch may be attached tothe output of light source 110 or a fiber optic output of the isolator115.

In an embodiment, the fiber network 166 of FIG. 3 is prior art fibernetwork 420B of FIG. 8B. Here the optical fibers 184, 186, 168, 169 ofFIG. 3 correspond to optical fibers 443, 444, 424, 422 of FIG. 8B. Thefiber network of FIG. 8B is like the fiber network of FIG. 8A exceptthat the fiber network of FIG. 8B has a single fiber coupler instead oftwo fiber couplers. The advantage of FIG. 8B over FIG. 8A is simplicity;however, FIG. 8B is more likely to have unwanted optical backreflections entering the optical fibers 422 and 424.

In an embodiment, the fiber network 166 of FIG. 3 is fiber network 420Cof FIG. 8C. Here the optical fibers 184, 186, 168, 169 of FIG. 3correspond to optical fibers 447, 455, 423, 424 of FIG. 8C. The fibernetwork 420C includes a first fiber coupler 445 and a second fibercoupler 451. The first fiber coupler 445 is a 2×2 coupler having twoinput ports and two output ports. Couplers of this type are usually madeby placing two fiber cores in close proximity and then drawing thefibers while heated. In this way, evanescent coupling between the fiberscan split off a desired fraction of the light to the adjacent fiber. Thesecond fiber coupler 451 is of the type called a circulator. It hasthree ports, each having the capability of transmitting or receivinglight, but only in the designated direction. For example, the light onoptical fiber 448 enters port 453 and is transported toward port 454 asindicated by the arrow. At port 454, light may be transmitted to opticalfiber 455. Similarly, light traveling on port 455 may enter port 454 andtravel in the direction of the arrow to port 456, where some light maybe transmitted to the optical fiber 424. If only three ports are needed,then the circulator 451 may suffer less losses of optical power than the2×2 coupler. On the other hand, a circulator 451 may be more expensivethan a 2×2 coupler, and it may experience polarization mode dispersion,which can be problematic in some situations.

FIGS. 9 and 10 show exploded and cross sectional views, respectively, ofa prior art laser tracker 2100, which is depicted in FIGS. 2 and 3 ofthe aforementioned patent '983. Azimuth assembly 2110 includes posthousing 2112, azimuth encoder assembly 2120, lower and upper azimuthbearings 2114A, 2114B, azimuth motor assembly 2125, azimuth slip ringassembly 2130, and azimuth circuit boards 2135.

The purpose of azimuth encoder assembly 2120 is to accurately measurethe angle of rotation of yoke 2142 with respect to the post housing2112. Azimuth encoder assembly 2120 includes encoder disk 2121 andread-head assembly 2122. Encoder disk 2121 is attached to the shaft ofyoke housing 2142, and read head assembly 2122 is attached to postassembly 2110. Read head assembly 2122 comprises a circuit board ontowhich one or more read heads are fastened. Laser light sent from readheads reflect off fine grating lines on encoder disk 2121. Reflectedlight picked up by detectors on encoder read head(s) is processed tofind the angle of the rotating encoder disk in relation to the fixedread heads.

Azimuth motor assembly 2125 includes azimuth motor rotor 2126 andazimuth motor stator 2127. Azimuth motor rotor comprises permanentmagnets attached directly to the shaft of yoke housing 2142. Azimuthmotor stator 2127 comprises field windings that generate a prescribedmagnetic field. This magnetic field interacts with the magnets ofazimuth motor rotor 2126 to produce the desired rotary motion. Azimuthmotor stator 2127 is attached to post frame 2112.

Azimuth circuit boards 2135 represent one or more circuit boards thatprovide electrical functions required by azimuth components such as theencoder and motor. Azimuth slip ring assembly 2130 includes outer part2131 and inner part 2132. In an embodiment, wire bundle 2138 emergesfrom auxiliary unit processor 50. Wire bundle 2138 may carry power tothe tracker or signals to and from the tracker. Some of the wires ofwire bundle 2138 may be directed to connectors on circuit boards. In theexample shown in FIG. 10, wires are routed to azimuth circuit board2135, encoder read head assembly 2122, and azimuth motor assembly 2125.Other wires are routed to inner part 2132 of slip ring assembly 2130.Inner part 2132 is attached to post assembly 2110 and consequentlyremains stationary. Outer part 2131 is attached to yoke assembly 2140and consequently rotates with respect to inner part 2132. Slip ringassembly 2130 is designed to permit low impedance electrical contact asouter part 2131 rotates with respect to the inner part 2132.

Zenith assembly 2140 comprises yoke housing 2142, zenith encoderassembly 2150, left and right zenith bearings 2144A, 2144B, zenith motorassembly 2155, zenith slip ring assembly 2160, and zenith circuit board2165.

The purpose of zenith encoder assembly 2150 is to accurately measure theangle of rotation of payload frame 2172 with respect to yoke housing2142. Zenith encoder assembly 2150 comprises zenith encoder disk 2151and zenith read-head assembly 2152. Encoder disk 2151 is attached topayload housing 2142, and read head assembly 2152 is attached to yokehousing 2142. Zenith read head assembly 2152 comprises a circuit boardonto which one or more read heads are fastened. Laser light sent fromread heads reflect off fine grating lines on encoder disk 2151.Reflected light picked up by detectors on encoder read head(s) isprocessed to find the angle of the rotating encoder disk in relation tothe fixed read heads.

Zenith motor assembly 2155 comprises azimuth motor rotor 2156 andazimuth motor stator 2157. Zenith motor rotor 2156 comprises permanentmagnets attached directly to the shaft of payload frame 2172. Zenithmotor stator 2157 comprises field windings that generate a prescribedmagnetic field. This magnetic field interacts with the rotor magnets toproduce the desired rotary motion. Zenith motor stator 2157 is attachedto yoke frame 2142.

Zenith circuit board 2165 represents one or more circuit boards thatprovide electrical functions required by zenith components such as theencoder and motor. Zenith slip ring assembly 2160 comprises outer part2161 and inner part 2162. Wire bundle 2168 emerges from azimuth outerslip ring 2131 and may carry power or signals. Some of the wires of wirebundle 2168 may be directed to connectors on circuit board. In theexample shown in FIG. 10, wires are routed to zenith circuit board 2165,zenith motor assembly 2150, and encoder read head assembly 2152. Otherwires are routed to inner part 2162 of slip ring assembly 2160. Innerpart 2162 is attached to yoke frame 2142 and consequently rotates inazimuth angle only, but not in zenith angle. Outer part 2161 is attachedto payload frame 2172 and consequently rotates in both zenith andazimuth angles. Slip ring assembly 2160 is designed to permit lowimpedance electrical contact as outer part 2161 rotates with respect tothe inner part 2162. Payload assembly 2170 includes a main opticsassembly 2180 and a secondary optics assembly 2190.

FIG. 11 is a block diagram depicting a dimensional measurementelectronics processing system 1500 that includes a laser trackerelectronics processing system 1510, processing systems of peripheralelements 1582, 1584, 1586, computer 1590, and other networked components1600, represented here as a cloud. Exemplary laser tracker electronicsprocessing system 1510 includes a master processor 1520, payloadfunctions electronics 1530, azimuth encoder electronics 1540, zenithencoder electronics 1550, display and user interface (UI) electronics1560, removable storage hardware 1565, radio frequency identification(RFID) electronics, and an antenna 1572. The payload functionselectronics 1530 includes a number of subfunctions including the six-DOFelectronics 1531 (for a six-DOF system such as that shown herein and inFIG. 2), the camera electronics 1532, the ADM electronics 1533, theposition detector (PSD) electronics 1534, and the level electronics1535. Most of the subfunctions have at least one processor unit, whichmight be a digital signal processor (DSP) or field programmable gatearray (FPGA), for example. The electronics units 1530, 1540, and 1550are separated as shown because of their location within the lasertracker. In an embodiment, the payload functions 1530 are located in thepayload 2170 of FIGS. 9 and 10, while the azimuth encoder electronics1540 is located in the azimuth assembly 2110 and the zenith encoderelectronics 1550 is located in the zenith assembly 2140.

Many types of peripheral devices are possible, but here three suchdevices are shown: a temperature sensor 1582, a six-DOF probe 1584, anda personal digital assistant, 1586, which might be a smart phone, forexample. As will be discussed in more detail hereinafter in accordancewith embodiments of the present invention, another type of peripheraldevice not shown in FIG. 11 is a triangulation scanner, in particular athree-DOF triangulation scanner, such as, for example, a laser lineprobe (LLP) or an area structured light scanner. The laser tracker maycommunicate with the peripheral devices in a variety of ways, includingby wireless communication over the antenna 1572, by a vision system suchas a camera, and by distance and angular readings of the laser trackerto a cooperative target such as the six-DOF probe 1584 or triangulationscanner. The peripheral devices may contain processors. Generally, whenthe term laser tracker processor or measurement device processor isused, it is meant to include possible external computer and cloudsupport.

In an embodiment, a separate communications bus goes from the masterprocessor 1520 to each of the electronics units 1530, 1540, 1550, 1560,1565, and 1570. Each communications line may have, for example, threeserial lines that include the data line, clock line, and frame line. Theframe line indicates whether or not the electronics unit should payattention to the clock line. If it indicates that attention should begiven, the electronics unit reads the current value of the data line ateach clock signal. The clock-signal may correspond, for example, to arising edge of a clock pulse. In an embodiment, information istransmitted over the data line in the form of a packet. In anembodiment, each packet includes an address, a numeric value, a datamessage, and a checksum. The address indicates where, within theelectronics unit, the data message is to be directed. The location may,for example, correspond to a processor subroutine within the electronicsunit. The numeric value indicates the length of the data message. Thedata message contains data or instructions for the electronics unit tocarry out. The checksum is a numeric value that is used to minimize thechance that errors are transmitted over the communications line.

In an embodiment, the master processor 1520 sends packets of informationover bus 1610 to payload functions electronics 1530, over bus 1611 toazimuth encoder electronics 1540, over bus 1612 to zenith encoderelectronics 1550, over bus 1613 to display and UI electronics 1560, overbus 1614 to removable storage hardware 1565, and over bus 1616 to RFIDand wireless electronics 1570.

In an embodiment, master processor 1520 also sends a synch(synchronization) pulse over the synch bus 1630 to each of theelectronics units at the same time. The synch pulse provides a way ofsynchronizing values collected by the measurement functions of the lasertracker. For example, the azimuth encoder electronics 1540 and thezenith electronics 1550 latch their encoder values as soon as the synchpulse is received. Similarly, the payload functions electronics 1530latch the data collected by the electronics contained within thepayload. The six-DOF, ADM, and position detector all latch data when thesynch pulse is given. In most cases, the camera and inclinometer collectdata at a slower rate than the synch pulse rate but may latch data atmultiples of the synch pulse period.

The azimuth encoder electronics 1540 and zenith encoder electronics 1550are typically separated from one another and from the payloadelectronics 1530 by slip rings, for example. This is why the bus lines1610, 1611, and 1612 are depicted as separate bus lines in FIG. 11.

The laser tracker electronics processing system 1510 may communicatewith an external computer 1590, or it may provide computation, display,and UI functions within the laser tracker. The laser trackercommunicates with computer 1590 over communications link 1606, whichmight be, for example, an Ethernet line or a wireless connection. Thelaser tracker may also communicate with other elements 1600, representedby the cloud, over communications link 1602, which might include one ormore electrical cables, such as Ethernet cables, and one or morewireless connections. An example of an element 1600 is anotherthree-dimensional test instrument—for example, an articulated arm CMM,which may be relocated by the laser tracker. A communication link 1604between the computer 1590 and the elements 1600 may be wired (e.g.,Ethernet) or wireless. An operator sitting on a remote computer 1590 maymake a connection to the Internet, represented by the cloud 1600, overan Ethernet or wireless line, which in turn connects to the masterprocessor 1520 over an Ethernet or wireless line. In this way, a usermay control the action of a remote laser tracker.

Laser trackers today use one visible wavelength (usually red) and oneinfrared wavelength for the ADM. The red wavelength may be provided by afrequency stabilized helium-neon (HeNe) laser suitable for use in aninterferometer and also for use in providing a red pointer beam.Alternatively, the red wavelength may be provided by a diode laser thatserves just as a pointer beam. A disadvantage in using two light sourcesis the extra space and added cost required for the extra light sources,beam splitters, isolators, and other components. Another disadvantage inusing two light sources is that it is difficult to perfectly align thetwo light beams along the entire paths the beams travel. This may resultin a variety of problems including inability to simultaneously obtaingood performance from different subsystems that operate at differentwavelengths. A system that uses a single light source, therebyeliminating these disadvantages, is shown in optoelectronic system 500of FIG. 12A.

FIG. 12A includes a visible light source 110, an isolator 115, a fibernetwork 420, ADM electronics 530, a fiber launch 170, a beam splitter145, and a position detector 150. The visible light source 110 might be,for example, a red or green diode laser or a vertical cavity surfaceemitting laser (VCSEL). The isolator might be a Faraday isolator, anattenuator, or any other device capable of sufficiently reducing theamount of light fed back into the light source. The light from theisolator 115 travels into the fiber network 420, which in an embodimentis the fiber network 420A of FIG. 8A.

FIG. 12B shows an embodiment of an optoelectronic system 400 in which asingle wavelength of light is used but wherein modulation is achieved byelectro-optic modulation of the light rather than by direct modulationof a light source. The optoelectronic system 400 includes a visiblelight source 110, an isolator 115, an electrooptic modulator 410, ADMelectronics 475, a fiber network 420, a fiber launch 170, a beamsplitter 145, and a position detector 150. The visible light source 110may be, for example, a red or green laser diode. Laser light is sentthrough an isolator 115, which may be a Faraday isolator or anattenuator, for example. The isolator 115 may be fiber coupled at itsinput and output ports. The isolator 115 sends the light to theelectrooptic modulator 410, which modulates the light to a selectedfrequency, which may be up to 10 GHz or higher if desired. An electricalsignal 476 from ADM electronics 475 drives the modulation in theelectrooptic modulator 410. The modulated light from the electroopticmodulator 410 travels to the fiber network 420, which might be the fibernetwork 420A, 420B, 420C, or 420D discussed hereinabove. Some of thelight travels over optical fiber 422 to the reference channel of the ADMelectronics 475. Another portion of the light travels out of thetracker, reflects off retroreflector 90, returns to the tracker, andarrives at the beam splitter 145. A small amount of the light reflectsoff the beam splitter and travels to position detector 150, which hasbeen discussed hereinabove with reference to FIGS. 6A-F. A portion ofthe light passes through the beam splitter 145 into the fiber launch170, through the fiber network 420 into the optical fiber 424, and intothe measure channel of the ADM electronics 475. In general, the system500 of FIG. 12A can be manufactured for less money than system 400 ofFIG. 12B; however, the electro-optic modulator 410 may be able toachieve a higher modulation frequency, which can be advantageous in somesituations.

FIG. 13 shows an embodiment of a locator camera system 950 and anoptoelectronic system 900 in which an orientation camera 910 is combinedwith the optoelectronic functionality of a 3D laser tracker to measuresix degrees of freedom of a device. The optoelectronic system 900includes a visible light source 905, an isolator 910, an optionalelectrooptic modulator 410, ADM electronics 715, a fiber network 420, afiber launch 170, a beam splitter 145, a position detector 150, a beamsplitter 922, and an orientation camera 910. The light from the visiblelight source is emitted in optical fiber 980 and travels throughisolator 910, which may have optical fibers coupled on the input andoutput ports. The light may travel through the electrooptic modulator410 modulated by an electrical signal 716 from the ADM electronics 715.Alternatively, the ADM electronics 715 may send an electrical signalover cable 717 to modulate the visible light source 905. Some of thelight entering the fiber network travels through the fiber lengthequalizer 423 and the optical fiber 422 to enter the reference channelof the ADM electronics 715. An electrical signal 469 may optionally beapplied to the fiber network 420 to provide a switching signal to afiber optic switch within the fiber network 420. A part of the lighttravels from the fiber network to the fiber launch 170, which sends thelight on the optical fiber into free space as light beam 982. A smallamount of the light reflects off the beamsplitter 145 and is lost. Aportion of the light passes through the beam splitter 145, through thebeam splitter 922, and travels out of the laser tracker to a sixdegree-of-freedom (DOF) device 4000. The six-DOF device 4000 may be aprobe, a projector, a sensor, or some other type of six-DOF device. Invarious embodiments, the six-DOF device 400 comprises a six-DOFtriangulation scanner such as, for example, a laser line probe (LLP) ora structured light area scanner. The six-DOF triangulation scanner has aretroreflector attached thereto to facilitate measuring or determiningof the six degrees of freedom of the scanner.

On its return path to the laser tracker, the light from the six-DOFdevice 4000 enters the optoelectronic system 900 and arrives atbeamsplitter 922. Part of the light is reflected off the beamsplitter922 and enters the orientation camera 910. The orientation camera 910records the positions of some marks placed on the retroreflector target.From these marks, the orientation angle (i.e., three degrees of freedom)of the six-DOF probe device is found. The principles of the orientationcamera are described in the aforementioned patent '758. A portion of thelight at beam splitter 145 travels through the beamsplitter and is putonto an optical fiber by the fiber launch 170. The light travels tofiber network 420. Part of this light travels to optical fiber 424, fromwhich it enters the measure channel of the ADM electronics 715.

The locator camera system 950 includes a camera 960 and one or morelight sources 970. The locator camera system is also shown with respectto the laser tracker 10 in FIG. 1, where the cameras are elements 52 andthe light sources are elements 54. The camera 960 includes a lens system962, a photosensitive array 964, and a body 966. One use of the locatorcamera system 950 is to locate retroreflector targets in the workvolume. It does this by flashing the light source 970, which the camerapicks up as a bright spot on the photosensitive array 964. A second useof the locator camera system 950 is establish a coarse orientation ofthe six-DOF device 4000 based on the observed location of a reflectorspot or LED on the six-DOF device 4000. If two or more locator camerasystems 950 are available on the laser tracker 10, the direction to eachretroreflector target in the work volume may be calculated using theprinciples of triangulation. If a single locator camera is located topick up light reflected along the optical axis of the laser tracker, thedirection to each retroreflector target may be found. If a single camerais located off the optical axis of the laser tracker, then approximatedirections to the retroreflector targets may be immediately obtainedfrom the image on the photosensitive array. In this case, a moreaccurate direction to a target may be found by rotating the mechanicalaxes of the laser to more than one direction and observing the change inthe spot position on the photosensitive array.

FIG. 14 shows an embodiment of a three-DOF triangulation scanner 2500used in conjunction with an optoelectronic system 900 and a locatorcamera system 950. The scanner 2500 may also be referred to as a “targetscanner.” The optoelectronic system 900 and the locator camera system950 were discussed in reference to FIG. 13. In another embodiment, theoptoelectronic system 900 is replaced by the optoelectronic system thatuses two or more wavelengths of light. The scanner 2500 includes a body2514, one or more retroreflectors 2510, 2511 a scanner camera 2530, ascanner light projector 2520, an optional electrical cable 2546, anoptional battery 2444, an interface component 2512, an identifierelement 2549, actuator buttons 2516, an antenna 2548, and an electronicscircuit board 2542. Together, the scanner projector 2520 and the scannercamera 2530 are used to measure the three-dimensional coordinates of aworkpiece 2528.

The three degrees of freedom of the scanner 2500 measured by the lasertracker may be considered to comprise three translational degrees offreedom. The three translational degrees of freedom may include a radialdistance measurement between the laser tracker and a retroreflector, afirst angular measurement, and a second angular measurement. The radialdistance measurement may be made with an IFM or an ADM within the lasertracker. The first angular measurement may be made with an azimuthangular measurement device, such as an azimuth angular encoder, and thesecond angular measurement made with a zenith angular measurementdevice, such as a zenith angular encoder. Alternatively, the firstangular measurement device may be the zenith angular measurement deviceand the second angular measurement device may be the azimuth angularmeasurement device. The radial distance, first angular measurement, andsecond angular measurement constitute three coordinates in a sphericalcoordinate system, which can be transformed into three coordinates in aCartesian coordinate system or another coordinate system.

The camera 2530 includes a camera lens system 2532 and a photosensitivearray 2534. The photosensitive array 2534 may be a CCD or CMOS array,for example. The scanner projector 2520 includes a projector lens system2523 and a source pattern of light 2524. The source pattern of light mayemit a point of light, a line of light, or a two-dimensional (2D)structured pattern of light, either coded or uncoded, as discussed inmore detail hereinafter. If the scanner emits a line of light, thescanner may be referred to as a “laser line probe” (LLP). On the otherhand, if the scanner emits a 2D structured pattern of light, the scannermay be referred to as a “structured light scanner.” The structured lightpattern may be one of various patterns such as, for example, thosepatterns disclosed in the journal article “DLP-Based Structured Light 3DImaging Technologies and Applications,” by Jason Geng, published in theProceedings of SPIE, Vol. 7932, which is incorporated herein byreference.

In general, there are two categories of structured light, namely codedand uncoded structured light. A common form of uncoded structured lightrelies on a striped pattern varying in a periodic manner along onedimension. These types of patterns are usually applied in a sequence toprovide an approximate distance to the object. Some uncoded patternembodiments, such as sinusoidal patterns for example, may providerelatively highly accurate measurements. However, for these types ofuncoded patterns to be effective, it is usually necessary for thescanner device and the object to be held stationary relative to eachother. Where the scanner device or the object are in motion (relative tothe other), then a coded pattern may be used. A coded pattern allows theimage to be analyzed using a single acquired image. Some coded patternsmay be placed in a particular orientation on the projector pattern (forexample, perpendicular to epipolar lines on the projector plane),thereby simplifying analysis of the three-dimensional surfacecoordinates based on a single image.

The light from projector 2520 is reflected from a surface of theworkpiece 2528 and the reflected light is received by the camera 2530.It should be appreciated that variations or features in the workpiecesurface, such as one or more protrusions for example, create distortionsin the structured pattern when the image of the pattern is captured bythe camera 2530. Since the pattern is formed by structured light, it ispossible in some instances for a controller to determine a one to onecorrespondence between the pixels in the emitted pattern and the pixelsin the imaged pattern. This enables triangulation principals to be usedto determine the coordinates of each pixel in the imaged pattern. Thecollection of 3D coordinates of the workpiece surface is sometimesreferred to as a point cloud. By moving the scanner 2500 over thesurface, such as with a spindle for example, a point cloud may becreated of the entire workpiece 2528.

If the scanner light source emits a point of light, the point may bescanned, for example, with a moving mirror, to produce a line or anarray of lines. If the scanner light source emits a line of light, theline may be scanned, for example, with a moving mirror, to produce anarray of lines. In an embodiment, the source pattern of light might bean LED, laser, or other light source reflected off a digital micromirrordevice (DMD) such as a digital light projector (DLP) from TexasInstruments, a liquid crystal device (LCD) or liquid crystal on silicon(LCOS) device, or it may be a similar device used in transmission moderather than reflection mode. The source pattern of light might also be aslide pattern, for example, a chrome-on-glass slide, which might have asingle pattern or multiple patterns, the slides moved in and out ofposition as needed. Additional retroreflectors, such as retroreflector2511, may be added to the first retroreflector 2510 to enable the lasertracker to track the scanner from a variety of directions, therebygiving greater flexibility in the directions to which light may beprojected by the projector 2500.

The scanner 2500 may be held by hand or mounted, for example, on atripod, an instrument stand, a motorized carriage, or a robot endeffector. The three-dimensional coordinates of the workpiece 2528 ismeasured by the scanner camera 2530 by using the principles oftriangulation. There are several ways that the triangulation measurementmay be implemented, depending on the pattern of light emitted by thescanner light source 2520 and the type of photosensitive array 2534. Forexample, if the pattern of light emitted by the scanner light source2520 is a line of light or a point of light scanned into the shape of aline and if the photosensitive array 2534 is a two-dimensional array,then one dimension of the two-dimensional array 2534 corresponds to adirection of a point 2526 on the surface of the workpiece 2528. Theother dimension of the two-dimensional array 2534 corresponds to thedistance of the point 2526 from the scanner light source 2520. Hence thethree-dimensional coordinates of each point 2526 along the line of lightemitted by scanner light source 2520 is known relative to the localframe of reference of the scanner 2500. From the three degrees offreedom, the three-dimensional coordinates of the scanned line of lightmay be found in the tracker frame of reference, which in turn may beconverted into the frame of reference of the workpiece 2528 through themeasurement by the laser tracker of three points on the workpiece, forexample.

If the scanner 2500 is held by hand, a line of laser light emitted bythe scanner light source 2520 may be moved in such a way as to “paint”the surface of the workpiece 2528, thereby obtaining thethree-dimensional coordinates for the entire surface. It is alsopossible to “paint” the surface of a workpiece using a scanner lightsource 2520 that emits a structured pattern of light. Alternatively,when using a scanner 2500 that emits a structured pattern of light, moreaccurate measurements may be made by mounting the scanner on a tripod orinstrument stand. The structured light pattern emitted by the scannerlight source 2520 might, for example, include a pattern of fringes, eachfringe having an irradiance that varies sinusoidally over the surface ofthe workpiece 2528. In an embodiment, the sinusoids are shifted by threeor more phase values. The amplitude level recorded by each pixel of thecamera 2530 for each of the three or more phase values is used toprovide the position of each pixel on the sinusoid. This information isused to help determine the three-dimensional coordinates of each point2526. In another embodiment, the structured light may be in the form ofa coded pattern that may be evaluated to determine three-dimensionalcoordinates based on single, rather than multiple, image framescollected by the camera 2530. Use of a coded pattern may enablerelatively accurate measurements while the scanner 2500 is moved by handat a reasonable speed.

Projecting a structured light pattern, as opposed to a line of light,has some advantages. In a line of light projected from a handheldscanner 2500 such as an LLP, the density of points may be high along theline but much less between the lines. With a structured light pattern,the spacing of points is usually about the same in each of the twoorthogonal directions. In addition, in some modes of operation, thethree-dimensional points calculated with a structured light pattern maybe more accurate than other methods. For example, by fixing the scanner2500 in place, for example, by attaching it to a stationary stand ormount, a sequence of structured light patterns may be emitted thatenable a more accurate calculation than would be possible other methodsin which a single pattern was captured (i.e., a single-shot method). Anexample of a sequence of structured light patterns is one in which apattern having a first spatial frequency is projected onto the object.In an embodiment, the projected pattern is a pattern of stripes thatvary sinusoidally in optical power. In an embodiment, the phase of thesinusoidally varying pattern is shifted, thereby causing the stripes toshift to the side. For example, the pattern may be made to be projectedwith three phase angles, each shifted by 120 degrees relative to theprevious pattern. This sequence of projections provides enoughinformation to enable relatively accurate determination of the phase ofeach point of the pattern, independent of the background light. This canbe done on a point by point basis without considering adjacent points onthe object surface.

Although the procedure above determines a phase for each point withphases running from 0 to 360 degrees between two adjacent lines, theremay still be a question about which line is which. A way to identify thelines is to repeat the sequence of phases, as described above, but usinga sinusoidal pattern with a different spatial frequency (i.e., adifferent fringe pitch). In some cases, the same approach needs to berepeated for three or four different fringe pitches. The method ofremoving ambiguity using this method is well known in the art and is notdiscussed further here.

To obtain the best possible accuracy using a sequential projectionmethod such as the sinusoidal phase-shift method described above, it maybe advantageous to minimize the movement of the scanner. Although theposition of the scanner is known from the measurements made by the lasertracker and although corrections can be made for movements of a handheldscanner, the resulting noise will be somewhat higher than it would havebeen if the scanner were kept stationary by placing it on a stationarymount, stand, or fixture.

The scanning methods represented by FIGS. 15A and 15B are based on theprinciple of triangulation. A more complete explanation of theprinciples of triangulation are given with reference to the system 2560of FIG. 15A and the system 4760 of FIG. 15B. Referring first to FIG.15A, the system 2560 includes a projector 2562 and a camera 2564. Theprojector 2562 includes a source pattern of light 2570 lying on a sourceplane and a projector lens 2572. The projector lens may include severallens elements. The projector lens has a lens perspective center 2575 anda projector optical axis 2576. The ray of light 2573 travels from apoint 2571 on the source pattern of light through the lens perspectivecenter onto the object 2590, which it intercepts at a point 2574.

The camera 2564 includes a camera lens 2582 and a photosensitive array2580. The camera lens 2582 has a lens perspective center 2585 and anoptical axis 2586. A ray of light 2583 travels from the object point2574 through the camera perspective center 2585 and intercepts thephotosensitive array 2580 at point 2581.

The line segment that connects the perspective centers is the baseline2588 in FIG. 15A and the baseline 4788 in FIG. 15B. The length of thebaseline is called the baseline length (2592, 4792). The angle betweenthe projector optical axis and the baseline is the baseline projectorangle (2594, 4794). The angle between the camera optical axis (2583,4786) and the baseline is the baseline camera angle (2596, 4796). If apoint on the source pattern of light (2570, 4771) is known to correspondto a point on the photosensitive array (2581, 4781), then it is possibleusing the baseline length, baseline projector angle, and baseline cameraangle to determine the sides of the triangle connecting the points 2585,2574, and 2575, and hence determine the surface coordinates of points onthe surface of object 2590 relative to the frame of reference of themeasurement system 2560. To do this, the angles of the sides of thesmall triangle between the projector lens 2572 and the source pattern oflight 2570 are found using the known distance between the lens 2572 andplane 2570 and the distance between the point 2571 and the intersectionof the optical axis 2576 with the plane 2570. These small angles areadded or subtracted from the larger angles 2596 and 2594 as appropriateto obtain the desired angles of the triangle. It will be clear to one ofordinary skill in the art that equivalent mathematical methods can beused to find the lengths of the sides of the triangle 2574-2585-2575 orthat other related triangles may be used to obtain the desiredcoordinates of the surface of object 2590.

Referring first to FIG. 15B, the system 4760 is similar to the system2560 of FIG. 15A except that the system 4760 does not include a lens.The system may include a projector 4762 and a camera 4764. In theembodiment illustrated in FIG. 15B, the projector includes a lightsource 4778 and a light modulator 4770. The light source 4778 may be alaser light source since such a light source may remain in focus for along distance using the geometry of FIG. 15B. A ray of light 4773 fromthe light source 4778 strikes the optical modulator 4770 at a point4771. Other rays of light from the light source 4778 strike the opticalmodulator at other positions on the modulator surface. In an embodiment,the optical modulator 4770 changes the power of the emitted light, inmost cases by decreasing the optical power to a degree. In this way, theoptical modulator imparts an optical pattern to the light, referred tohere as the source pattern of light, which is at the surface of theoptical modulator 4770. The optical modulator 4770 may be a DLP or LCOSdevice for example. In some embodiments, the modulator 4770 istransmissive rather than reflective. The light emerging from the opticalmodulator 4770 appears to emerge from a virtual light perspective center4775. The ray of light appears to emerge from the virtual lightperspective center 4775, pass through the point 4771, and travel to thepoint 4774 at the surface of object 4790.

The baseline is the line segment extending from the camera lensperspective center 4785 to the virtual light perspective center 4775. Ingeneral, the method of triangulation involves finding the lengths of thesides of a triangle, for example, the triangle having the vertex points4774, 4785, and 4775. A way to do this is to find the length of thebaseline, the angle between the baseline and the camera optical axis4786, and the angle between the baseline and the projector referenceaxis 4776. To find the desired angle, additional smaller angles arefound. For example, the small angle between the camera optical axis 4786and the ray 4783 can be found by solving for the angle of the smalltriangle between the camera lens 4782 and the photosensitive array 4780based on the distance from the lens to the photosensitive array and thedistance of the pixel from the camera optical axis. The angle of thesmall triangle is then added to the angle between the baseline and thecamera optical axis to find the desired angle. Similarly for theprojector, the angle between the projector reference axis 4776 and theray 4773 is found can be found by solving for the angle of the smalltriangle between these two lines based on the known distance of thelight source 4777 and the surface of the optical modulation and thedistance of the projector pixel at 4771 from the intersection of thereference axis 4776 with the surface of the optical modulator 4770. Thisangle is subtracted from the angle between the baseline and theprojector reference axis to get the desired angle.

The camera 4764 includes a camera lens 4782 and a photosensitive array4780. The camera lens 4782 has a camera lens perspective center 4785 anda camera optical axis 4786. The camera optical axis is an example of acamera reference axis. From a mathematical point of view, any axis thatpasses through the camera lens perspective center may equally easily beused in the triangulation calculations, but the camera optical axis,which is an axis of symmetry for the lens, is customarily selected. Aray of light 4783 travels from the object point 4774 through the cameraperspective center 4785 and intercepts the photosensitive array 4780 atpoint 4781. Other equivalent mathematical methods may be used to solvefor the lengths of the sides of a triangle 4774-4785-4775, as will beclear to one of ordinary skill in the art.

Although the triangulation method described here is well known, someadditional technical information is given hereinbelow for completeness.Each lens system has an entrance pupil and an exit pupil. The entrancepupil is the point from which the light appears to emerge, whenconsidered from the point of view of first-order optics. The exit pupilis the point from which light appears to emerge in traveling from thelens system to the photosensitive array. For a multi-element lenssystem, the entrance pupil and exit pupil do not necessarily coincide,and the angles of rays with respect to the entrance pupil and exit pupilare not necessarily the same. However, the model can be simplified byconsidering the perspective center to be the entrance pupil of the lensand then adjusting the distance from the lens to the source or imageplane so that rays continue to travel along straight lines to interceptthe source or image plane. In this way, the simple and widely used modelshown in FIG. 15A is obtained. It should be understood that thisdescription provides a good first order approximation of the behavior ofthe light but that additional fine corrections can be made to accountfor lens aberrations that can cause the rays to be slightly displacedrelative to positions calculated using the model of FIG. 15A. Althoughthe baseline length, the baseline projector angle, and the baselinecamera angle are generally used, it should be understood that sayingthat these quantities are required does not exclude the possibility thatother similar but slightly different formulations may be applied withoutloss of generality in the description given herein.

When using a triangulation scanner, several types of scan patterns maybe used, and it may be advantageous to combine different types to obtainthe best performance in the least time. For example, in an embodiment, afast measurement method uses a two-dimensional coded pattern in whichthree-dimensional coordinate data may be obtained in a single shot. In amethod using coded patterns, different characters, different shapes,different thicknesses or sizes, or different colors, for example, may beused to provide distinctive elements, also known as coded elements orcoded features. Such features may be used to enable the matching of thepoint 2571 to the point 2581. A coded feature on the source pattern oflight 2570 may be identified on the photosensitive array 2580.

A technique that may be used to simplify the matching of coded featuresis the use of epipolar lines. Epipolar lines are mathematical linesformed by the intersection of epipolar planes and the source plane 2570or the image plane 2580. An epipolar plane is any plane that passesthrough the projector perspective center and the camera perspectivecenter. The epipolar lines on the source plane and image plane may beparallel in some special cases, but in general are not parallel. Anaspect of epipolar lines is that a given epipolar line on the projectorplane has a corresponding epipolar line on the image plane. Hence, anyparticular pattern known on an epipolar line in the projector plane maybe immediately observed and evaluated in the image plane. For example,if a coded pattern is placed along an epipolar line in the projectorplane that the spacing between coded elements in the image plane may bedetermined using the values read out by pixels of the photosensitivearray 2580 and this information used to determine the three-dimensionalcoordinates of an object point 2574. It is also possible to tilt codedpatterns at a known angle with respect to an epipolar line andefficiently extract object surface coordinates.

An advantage of using coded patterns is that three-dimensionalcoordinates for object surface points can be quickly obtained. However,in most cases, a sequential structured light approach, such as thesinusoidal phase-shift approach discussed above, will give more accurateresults. Therefore, the user may advantageously choose to measurecertain objects or certain object areas or features using differentprojection methods according to the accuracy desired. By using aprogrammable source pattern of light, such a selection may easily bemade.

According to embodiments of the present invention, the triangulationscanner 2500 of FIG. 14 also includes an augmented reality (AR) camera2560. The AR camera 2560 may be considered to be one that is capable oftaking “full field” images. The AR camera 2560 includes a camera lens2562 and a photosensitive array 2564. The photosensitive array 2564 maybe a CCD or CMOS array, for example. Thus, the AR camera 2560 may bedigital in nature, and may take still images or video images.

As shown in FIG. 14, the AR camera 2560 is connected by dashed lines2566 to the body 2514 of the triangulation scanner 2500. However, thisis for illustrative purposes only. The AR camera 2560 may be an integralpart of the scanner body 2014. As used herein, the term “integral” isused to mean that the AR camera 2560 may be permanently or temporarilyattached to the scanner body 2014 such that the AR camera 2560 is in afixed spatial relationship with respect to the scanner body 2014. Eitherway, since the three degrees of freedom of the scanner 2500 are known asdescribed above, the three degrees of freedom of the AR camera 2560 arealso known for each image taken by the AR camera 2560. As such, thelaser tracker, the triangulation scanner 2500, and the AR camera 2560may all be placed into a common frame of reference.

Associated with the lens 2562 (which may be a lens system including aplurality of lens elements) is a perspective center of the lens. Therays of light passing through the lens 2562 may be considered to passthrough the perspective center before arriving at the photosensitivearray 2564. In a careful analysis, the lens 2562 may be characterized toaccount for lens aberrations, which result in a slight shift in theintersection positions of the rays on the photosensitive array 2564.However, without losing generality, it is possible to say that the rayspass through the perspective center, with aberration correction to theimage provided in another step of image processing.

The surface of an object under investigation is imaged by the lens 2562onto the photosensitive array 2564 to form an image on the collection ofpixels that are a part of the photosensitive array 2564. Light fallingon each pixel is converted, within an integration period of the camera,from a charge into a digital signal. An analog-to-digital converter,either located within the photosensitive array 2564 (for CMOS arrays) orexternal to the array 2564 (for CCD arrays), performs the conversionfrom analog to digital signal. The signal for each pixel is typicallygiven in a binary representation of between 8 and 12 bits. The 1's and0's represented by these bits are delivered over parallel channels, andmay be converted into serial form using a serializer/deserializercapability for transmission over a bus line.

As discussed, the triangulation scanner 2500 is handheld in embodimentsof the present invention. However, in other embodiments the scanner 2500may be kept stationary by placing it on a stationary mount, stand, orfixture; for example a tripod. Further, although the position andorientation of the triangulation scanner 2500 are known from themeasurements made by the laser tracker as described hereinabove andalthough corrections can be made for movements of a handheld scanner2500, the resulting noise may be somewhat greater than it would havebeen if the scanner 2500 were kept stationary. It is also possible tomount the triangulation scanner 2500 on a robot or machine tool.

In embodiments of the present invention, multiple two-dimensional (2D)camera images taken by an augmented reality camera 2560 that is part ofa triangulation scanner 2500 (which itself is used in conjunction with alaser tracker) are combined or “registered” together according to amethod, described hereinbelow, to obtain a three-dimensional (3D) imagerepresentation of various real-world features such as, for example, asurface of an object or of some real-world scene (e.g., the inside of abuilding, the location of a vehicle accident, or a crime scene). Thismethod is based on the fact that because the three degrees of freedom ofthe scanner 2500 having the integral AR camera 2560 is known for each 2Dphotograph or image taken by the AR camera 2560, a plurality of the 2Dphotographic images taken by the AR camera 2560 may be combined togetherto form a 3D image.

A method according to this embodiment is now described with reference tothe method 1600 of FIG. 16. In a step 1605, a triangulation scanner 2500and a coordinate measurement device are provided. The scanner 2500 alsoincludes the retroreflector 2510 and an integral AR camera 2560, asdescribed hereinabove with respect to FIG. 14. The coordinatemeasurement device, which may comprise a laser tracker such as the lasertracker 5 described hereinabove, has a device frame of reference and isseparate from the triangulation scanner 2500. The coordinate measurementdevice includes a first and second motor, a first and second anglemeasuring device, a distance meter, a position detector, a controlsystem, and a processor. The laser tracker and scanner with AR cameramay be identical or similar to those described and illustratedhereinabove.

A step 1610 is, in a first instance, measuring with the device the twoangles of rotation and distance to the retroreflector. In this step, a2D image is also formed on the AR camera 2560. The electronics circuitboard 2542 within the scanner 2500 may process and/or send the positioninformation from the AR camera 2560. The electronics circuit board 2542may also receive a first digital signal representing a 2D image sentthrough the camera lens 2562 onto the photosensitive array 2564.

In a step 1615, the triangulation scanner 2500 is moved to a newposition, and the device measures the two angles and distance to thesix-DOF scanner 2500. It also forms a 2D image at the new position. Theelectronics circuit board 2542 may process and/or send the positioninformation from the AR camera 2560 at this second position to the lasertracker. The electronics circuit board 2542 may also receive a seconddigital signal representing the 2D image sent through the camera lens2562 onto the photosensitive array 2564.

In a step 1620, a cardinal point in common to the first image and asecond image is found. The term “cardinal point” is typically used torefer to points that are identified in images and that can be used toconnect or register the images together. Also, these points aretypically not placed intentionally at their locations by someone. Thestep includes determining the corresponding locations of a cardinalpoint on a photosensitive array in first and second instances. We mayrefer to the location of the cardinal point in the first instance as thefirst location and the location of the cardinal point in the secondinstance as the second location. There is a well-developed collection oftechniques that may be used to determine such cardinal points, generallyusing methods referred to as image processing or feature detection. Acommonly used but general category for finding cardinal points isreferred to as interest point detection, with the points detectedreferred to as interest points. According to the usual definition, aninterest point has a mathematically well-founded definition, awell-defined position in space, an image structure around the interestpoint that is rich in local information content, and a variation inillumination level that is relatively stable over time. A particularexample of an interest point is a corner point, which might be a pointcorresponding to an intersection of three planes, for example. Anotherexample of signal processing that may be used is scale invariant featuretransform (SIFT), which is a method well known in the art and describedin U.S. Pat. No. 6,711,293 to Lowe. In the step 1620, the processorfinds those cardinal points common to the first and second images toobtain at least one cardinal point (but usually a large set of cardinalpoints). Other common feature detection methods for finding cardinalpoints include edge detection, blob detection, and ridge detection.

A step 1625 is determining the 3D coordinates of the correspondingcardinal points in the first and second 2D images within a first frameof reference. This determination may be made by first usingtriangulation to determine the 3D coordinates of the cardinal points inthe first and second images in the frame of reference of the scanner2500 and then using coordinate transformations to obtain the cardinalpoints in a first frame of reference, which might be the device frame ofreference. These coordinate transformations are based at least in parton the two angle measurements and one distance measurement provided bythe device. It is understood that, in this calculation, the position ofthe 2D camera 2560 within the scanner 2500 is known, for example, basedon factory measurements. Based on the measured values, a baseline havinga baseline distance may be drawn between the perspective centers of thecamera in a first instance (corresponding to the first 2D image) and ina second instance (corresponding to the second 2D image). Because thebaseline distance is known from the measurements by the tracker, eachcardinal point is determined in 3D space in the first frame ofreference. In general, the 3D coordinates of a cardinal point in thefirst image and the 3D coordinates of a corresponding cardinal point inthe second image will not match exactly. However, averaging of the 3Dcoordinates or similar methods may be used to determine a representative3D coordinate in the first frame of reference for the correspondingcardinal points in the first and second 2D images. If this method isused, then the 3D coordinates are properly scaled, which is to say that3D coordinates have the correct units of length within the first frameof reference. This is in contrast to a method of obtaining two imageswith a camera held in two different positions for which the positions ofthe camera perspective center in each case is unknown. This method doesnot provide enough information to properly scale a 3D image (in units oflength).

A step 1630 is creating a composite 3D image based at least in part onthe first and second 2D images and on the 3D coordinates of the cardinalpoints in the first frame of reference. In most cases, the first andsecond 2D images will share many cardinal points, and for each of these3D coordinates can be obtained. These cardinal points form the frameworkfor a 3D representation onto which other image elements may beinterpolated between the two 2D images or among multiple 2D images (in amore general case). Besides providing 3D coordinate information, acomposite 3D image may also convey texture and color informationobtained not only from cardinal points but also from visible regionsbetween the cardinal points, again using interpolation methods. The 3Dcoordinates obtained in the step 1625 and 1630 are the same for thecorresponding cardinal points in the first and second images; however,the composite 3D image may contain additional 3D information, forexample, by interpolation between cardinal points, and by adding colorand texture information.

A step 1635 is storing the composite 3D image.

Although not included in the procedure 1600 of FIG. 16, it is clear thatthe method described hereinabove may be extended to an arbitrarily largenumber of photosensitive array images so that sets of cardinal pointsmay be obtained from multiple images. In this case, each cardinal pointmay correspond to cardinal points on several of the images obtained withthe photosensitive array 2564 in different positions of the AR camera2560. For a given cardinal point, the points of intersection of themultiple lines projected from the photosensitive array 2564 through theperspective centers of the camera lens 2562 may be determined using abest-fit method according to methods of optimization well known in theart, for example, by using least-squares minimization methods.Additional optimization of registration of the multiple images may becarried out, if desired, by providing targets on or near the objectunder test. Additional targets may be reflective targets or lightemitting diodes, for example.

If the AR camera 2560 is a color camera, the reconstructed 3D surfacemay be represented in color, or other texture attributes may beretrieved. Various features of a light pattern, in addition to a 3Dsurface profile, may be provided by this method 1600 in otherembodiments. For example, an “X” marked on the surface of an object maybe retrieved in addition to the general coordinates corresponding to thelocation of the “X.”

In some cases, it may be known ahead of time that certain portions ofsurfaces being photographed are relatively smooth. In other words, theseportions do not have any sharp discontinuities or fine features. Inthese cases, it may be possible to use the established cardinal pointsto construct an unmeasured portion of the surface in three dimensions.For example, the cardinal points may fit smoothly onto a cylindricalshape over a portion of the surface, and so software may automaticallyprovide the cylindrical shape.

In the case that an overall shape for a portion of a surface is known,it may be possible to project a captured image onto the surface. Forexample, suppose that the surface has a colored pattern that may beprojected onto an assumed surface, which in a particular case might be aflat surface, for example. In this case, this pattern may be projectedonto the assumed surface from each of images obtained for the AR camera2560 in different positions. In this instance, the images would beexpected to overlap on the surface. If this is not the case, it wouldindicate that the assumed shape is not correct, and a change to theshape should be made. In this instance, it may be a good practice toobtain additional cardinal points based on the images captured by the ARcamera 2560 in the different positions. These additional cardinal pointsmay then be used to more accurately determine the surface profile.

An AR camera 2560 may be used to capture background images, for exampleof relatively distant background objects over a relatively large fieldof view and it may also be used to capture foreground images, which maybe, for example, images of an object being scanned by the triangulationscanner 2500.

As can be seen from the foregoing “dynamic triangulation” method 1600 asillustrated in FIG. 16, the images taken by the AR camera 2560 atvarious camera positions are registered together based in part onknowledge of the three degrees of freedom provided by the laser tracker.As such, it is possible to register the 2D images from the AR camera2560 with proper dimensional scale and with fewer camera images thatwould otherwise be possible.

Once the 3D images have been created by embodiments of the presentinvention, these images may have data overlaid or superimposed thereon.For example, if the 3D images are those of an object being built oralready built, the data superimposed on the 3D images may comprise CADdesign data of the object. The CAD data may be stored in memoryassociated with the laser tracker 5 (FIG. 1). Other types of data may besuperimposed on the camera images such as, for example, marks toindicate where various assembly operations (drilling, attaching, etc.)are to be performed.

An AR camera 2562 in a triangulation scanner 2500 may be used to measuresurroundings instead of (or in addition to) a part being measured withthe scanner 2500. For example, the AR camera 2560 may have a relativelylong focal length that enables it to clearly image its surroundings to arelatively greater extent that the camera 2530 associated with thetriangulation scanner 2500 is able to image. Reconstruction methodsdescribed hereinabove may be used to obtain a 3D representation of thesurroundings based on the AR camera images. For example, one or moreparts measured to an accuracy of a few micrometers or tens ofmicrometers with the scanner 2500 may be placed within surroundingsmeasured to an accuracy of a few millimeters based on the AR cameraimages.

Software may be used to observe the object and the surroundings fromdifferent perspectives and different distances, with the parallax shiftbetween the object and surroundings properly represented. In some cases,the background information may be important. For example, a project mayinvolve attaching a structure to the object being measured whileconfirming that there is adequate room in the 3D surroundings having a3D image obtained with the AR camera 2560. Such a structure may beavailable as a CAD model, as a scanned image of a part or assembly, oras a scaled 3D representation obtained through the use of multiplecamera images.

In some cases, the AR camera 2560 may be used to obtain representationsof areas ordinarily obstructed from view. For example, the AR camera maybe used to view all sides of an object to obtain 3D images of regionsnot easily measured with a scanner 2500. Such full coverage from alldirections is particularly useful when images are displayed—for example,in a presentation, on a website, or in a brochure. The addition of color(texture) from the AR camera 2560 also of value in this instance. 3Drepresentations obtained from the AR camera 2560 may be supplemented byother 3D representations. Models of parts, assemblies, furniture, and soforth, may in some cases be downloaded from files or websites andincorporated into a composite 3D representation.

Another important use for the AR camera 2030 and triangulation scanner2500 is to obtain proper scaling of surroundings. For example, a wallmay have a left side, a right side, an upper side, and a lower side.Although the method of matching cardinal points described hereinaboveprovides scaled 3D images, the dimensional accuracy will generally bemuch better is 3D coordinates are measured with the scanner 2500 thanwith camera images alone. By combining the composite 3D image obtainedfrom the 2D AR camera images with a few measurements with thetriangulation scanner 2500, the scaling accuracy of the composite 3Dimage can, in many cases, be greatly improved. For example, improvedscale of a building may be obtained by measuring one or more positionson each of the left, right, upper, and lower sides with the scanner2500.

The AR camera 2560 may be used to measure only surroundings, onlyobjects, or both surroundings or objects. As the term is used here, theword “object” means an item for which accurate dimensional informationis desired. An object is typically measured by the triangulation scanner2500 which has accuracy on the order of a few tens of micrometers.Measurement by an AR camera 2560 provides the ability to superimposeimages on drawings (for example, CAD). In addition, by obtaining 2Dimages of an object from multiple directions, it is possible to providean overlay to an object from all directions.

An object may be placed within its surroundings, the 3D coordinates ofwhich are obtained through the use of the AR camera 2560. With theinformation provided by the AR camera and triangulation scanner 2500, itis possible to view the objects from a variety of perspectives relativeto its surroundings and also to view an object or its surroundings fromall directions.

In an embodiment, a purely graphical element (which could be aphotographic element, a drawn element, or a rendered element, forexample) is placed within a composite image. A first example of such agraphical element is an addition to a machine tool on a factory floor.Such an addition may be superimposed on a CAD model to which a compositecolor image is overlaid. The addition might be a new machined part. Acollection of such additions may be placed in the context of a factoryenvironment to ensure that all elements fit properly. A second exampleof such a graphical element is a new item of machinery or furnitureplaced in the same factory environment. A question might be whether suchan element will fit in the new plans. In some cases, web sites may beavailable that enable downloading of such 3D images from the Cloud,which is a network typically found on the Internet through a serviceprovider. With some user interfaces, such a 3D component may be movedinto position with a computer mouse and then viewed from differentpositions and orientations.

While the invention has been described with reference to exampleembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Moreover, the use of the terms first, second, etc. do not denoteany order or importance, but rather the terms first, second, etc. areused to distinguish one element from another. Furthermore, the use ofthe terms a, an, etc. do not denote a limitation of quantity, but ratherdenote the presence of at least one of the referenced item.

What is claimed is:
 1. A method of combining a plurality oftwo-dimensional (2D) images into a three-dimensional (3D) image of anobject, the method comprising steps of: providing a six-degree offreedom (six-DOF) triangulation scanner having a retroreflector and anintegral augmented reality (AR) camera; providing a coordinatemeasurement device having a device frame of reference, the deviceincluding a distance meter and a processor, the device operable todirect a beam of light to a direction and measure a first angle ofrotation about a first axis and a second angle of rotation about asecond axis, the distance meter operable to measure a distance from thedevice to the retroreflector based at least in part on a portion of thebeam of light being reflected by the retroreflector and received by anoptical detector and on a speed of light in air, the processorconfigured to determine, in the device frame of reference, a 3Dcoordinates of the triangulation scanner; determining with the device afirst set of 3D coordinates of the triangulation scanner at a firstposition; forming a first 2D image with the camera at the firstposition; moving the triangulation scanner to a second position;determining with the device a second set of 3D coordinates of thetriangulation scanner at the second position; forming a second 2D imagewith the camera at the second position; determining a first cardinalpoint in common between the first and second 2D images, the firstcardinal point having a first location on the first 2D image and asecond location on the second 2D image; determining 3D coordinates ofthe first cardinal point in a first frame of reference based at least inpart on the first set of 3D coordinates, the second set of 3Dcoordinates, the first location, and the second location; creating the3D image as a first composite 3D image from the first 2D image and thesecond 2D image based at least in part on the first 2D image, the second2D image, and the 3D coordinates of the first cardinal point in thefirst frame of reference; and storing in a memory the first composite 3Dimage.
 2. The method of step 1, wherein the triangulation scannercomprises one of a laser line probe and a structured area scanner. 3.The method of claim 1, further including a step of measuring a firstplurality of points on a surface of the object.
 4. The method of claim3, further including a step of forming a first graphical representation,the first graphical representation including the first composite 3Dimage and the first plurality of points on the surface of the object,the first plurality of points viewed from a first observer perspectiverelative to the first composite 3D image.
 5. The method of claim 4,further including a step of forming a second graphical representation,the second graphical representation including the first composite 3Dimage and the first plurality of points on the surface of the object,the first plurality of points viewed form a second observer perspectiverelative to the first composite 3D image, the second observerperspective being different than the first observer perspective.
 6. Themethod of claim 1, wherein: in the step of forming the first 2D imagewith the camera, the first 2D image is a color image; in the step offorming the second 2D image with the camera, the second 2D image is acolor image; and in the step of creating the first composite 3D image,the first composite 3D image is a color image.
 7. The method of claim 3,wherein, in the step of forming a first 2D image with the camera, thefirst 2D image includes a first area of the surface of the object. 8.The method of claim 7, further including: a step of providing a drawing;and a step of superimposing the first composite 3D image onto thedrawing.
 9. The method of claim 1, further including: providing a firstgraphical element; and superimposing the first graphical element on thefirst composite 3D image.
 10. The method of claim 9, wherein, in thestep of providing a first graphical element, the first graphical elementis a mechanical extension of the object.
 11. The method of claim 9,wherein, in the step of providing a first graphical element, the firstgraphical element is placed within a volume represented by the firstcomposite 3D image.
 12. The method of claim 9, wherein, in the step ofproviding a first graphical element, the first graphical element isdownloaded from a network connection.
 13. The method of claim 7, furtherincludes: a step of forming a third 2D image with the camera, the third2D image including a second area of the surface of the object, thesecond area including a region of the object not visible in the firstcomposite 3D image; and a step of forming a second composite 3D image,the second composite 3D image including the third 2D image and at leasta portion of the first 2D image or a portion of the second 2D image. 14.A system for combining a plurality of two-dimensional (2D) images into athree-dimensional (3D) image of an object, the system comprising: athree-degree of freedom (six-DOF) triangulation scanner having aretroreflector and an integral augmented reality (AR) camera; acoordinate measurement device having a device frame of reference, thedevice including a distance meter and a processor, the device operableto direct a beam of light to a direction and measure a first angle ofrotation about a first axis and a second angle of rotation about asecond axis, the distance meter operable to measure a distance from thedevice to the retroreflector based at least in part on a portion of thebeam of light being reflected by the retroreflector and received by anoptical detector and on a speed of light in air, the processorconfigured to determine, in the device frame of reference, a 3Dcoordinates of the triangulation scanner; one or more processorsresponsive to executable computer instructions, the one or moreprocessors electrically coupled to the triangulation scanner and thedevice, the executable computer instructions comprising: determiningwith the device a first set of 3D coordinates of the triangulationscanner at a first position; forming a first 2D image with the camera atthe first position; moving the triangulation scanner to a secondposition; determining with the device a second set of 3D coordinates ofthe triangulation scanner at the second position; forming a second 2Dimage with the camera at the second position; determining a firstcardinal point in common between the first and second 2D images, thefirst cardinal point having a first location on the first 2D image and asecond location on the second 2D image; determining 3D coordinates ofthe first cardinal point in a first frame of reference based at least inpart on the first set of 3D coordinates, the second set of 3Dcoordinates, the first location, and the second location; creating the3D image as a first composite 3D image from the first 2D image and thesecond 2D image based at least in part on the first 2D image, the second2D image, and the 3D coordinates of the first cardinal point in thefirst frame of reference; and storing in a memory the first composite 3Dimage.
 15. The system of claim 14, wherein, in the step of forming afirst 2D image with the camera, the first 2D image includes a first areaof a surface of the object.
 16. The system of claim 15, furtherincluding: a step of providing a drawing; and a step of superimposingthe first composite 3D image onto the drawing.
 17. The system of claim14, further including: providing a first graphical element; andsuperimposing the first graphical element on the first composite 3Dimage.
 18. The system of claim 17, wherein, in the step of providing afirst graphical element, the first graphical element is a mechanicalextension of the object.
 19. The system of claim 17, wherein, in thestep of providing a first graphical element, the first graphical elementis placed within a volume represented by the first composite 3D image.20. The system of claim 15, further includes: a step of forming a third2D image with the camera, the third 2D image including a second area ofthe surface of the object, the second area including a region of theobject not visible in the first composite 3D image; and a step offorming a second composite 3D image, the second composite 3D imageincluding the third 2D image and at least a portion of the first 2Dimage or a portion of the second 2D image.